The present invention relates to implantable tissue stimulation systems, and more particularly to the independent generation of compliance voltages provided to each stimulation channel in an implantable multichannel tissue stimulation system such as a Spinal Cord Stimulation (SCS) system. A spinal cord stimulation system treats chronic pain by providing electrical stimulation pulses through the electrodes of an electrode array placed epidurally near a patient's spine. The electrode array is partitioned into channels including a current control circuit and cooperating electrodes. The level of stimulation in each channel is controlled by the current control circuit, and any excess power provided to a simulation channel is dissipated. Therefore, the independent generation of the compliance voltage provided to each stimulation channel results in efficient use of power by all of the stimulation channels.
Spinal cord stimulation is a well accepted clinical method for reducing pain in certain populations of patients. SCS systems typically include an Implantable Pulse Generator (IPG), an electrode array with attached electrode lead, and a lead extension. The IPG generates electrical pulses that are delivered to the dorsal column fibers within the spinal cord through the electrodes. The electrodes are implanted along the dura of the spinal cord. Individual electrode contacts (the “electrodes”) are arranged in a desired pattern and spacing in order to create an electrode array. Individual wires, within the electrode lead and lead extension, connect the IPG to each electrode in the array. The electrode lead exits the spinal cord and attaches to one or more lead extensions. The lead extension, in turn, is typically tunneled around the torso of the patient to a subcutaneous pocket where the IPG is implanted.
Spinal cord and other stimulation systems are known in the art. For example, an implantable electronic stimulator is disclosed in U.S. Pat. No. 3,646,940 that provides timed sequenced electrical impulses to a plurality of electrodes. As another example, U.S. Pat. No. 3,724,467 teaches an electrode implant for neuro-stimulation of the spinal cord. A relatively thin and flexible strip of biocompatible material is provided as a carrier on which a plurality of electrodes are formed. The electrodes are connected by a conductor, e.g., a lead body, to an RF receiver, which is also implanted, and which is controlled by an external controller.
The electrodes of an SCS system are grouped and included in stimulation channels. Most commonly, each channel includes two electrodes. The resistance of each channel is measured, and a compliance voltage for each channel is determined based on the measured resistance times the desired stimulation current. The resistances and stimulation currents of the channels may vary widely, and thus the compliance voltages also vary.
Known SCS systems include a single voltage source for all of the stimulation channels, and an independent current control circuit for each channel. The current control circuits are controlled by a stimulation control circuit to provide the correct current level to each channel. The voltage provided to each current control circuit is based on the requirements of the of the channel requiring the highest compliance voltage. In each channel that requires a lower voltage level, the excess power is dissipated within the current control circuit. The power dissipation represents a waste of power and places a burden on the battery powering the implantable device. Such burden on the battery results in a shortening of the battery life, and hastens the surgery required to replace the battery or device.
What is needed is a simple and efficient method of adjusting the compliance voltage provided to each channel, so as to avoid unnecessary power dissipation.